DX IS a huge time investment to complete. And it does run like crap on non-3DFx vidcards.
Here's my equation: C533A@825 + 128MB RAM + GF2 64MB@220/400 = ~25 FPS @ 1024x768/16Bit(!)/High Details, and that's after the D3D patch which did SQUAT for me. That's pretty inexcusable performance.
Performance issues aside it's still a good, but not great game. Just don't be fooled that there's really that much freedom involved. It's still basically a linear RPG game with some side quests and...
***SPOILER ALERT***
The 3 endings can all be accomplished no matter what "path" you take. They're all strictly dependent on what you do in the last level.
***END SPOILER ALERT***
If you accidentally read that, don't get pissed. You'll figure that out by the time you get to the end anyway. I just think the ways in which the the game has been promoted and often reviewed are deceptive about how open the gameworld and outcomes really are. You basically can do things three ways, via Stealth, Brute Force, or Information Gathering, or of course a limited combination of those three. This does "open-up" the game world, but it also leads to a lot of plot and design contrivances in order to allow you to play the game "your" way. Most of these things really took me out of the game. Everytime you had to interact with a NPC I felt like I got ripped out of the character because the view shifts to 3rd person and I had to listen to JC's annoying monotone. And things like finding a password laying around for everyone to read, for a computer you just hacked 5 minutes ago, is kind of inane game design, in my opinion. All this stuff was done SO much better in System Shock 2.
Anyway, if you want more straight-up action and not have to interact with NPCs except to kill them, then EF is more appropriate.